Fluorocarbons, or flon, have been widely used as refrigerants for freezers and refrigerators, however, some fluorocarbons containing chlorine destroy the ozone layer in the upper atmosphere. Therefore, as alternates for them, chlorine-free fluorocarbon or hydrocarbon refrigerants are required.
However, most of the chlorine-free fluorocarbons significantly absorb long-wavelength infrared rays. This causes global warming. Therefore, it is required to use as small amount as possible of materials showing minimal greenhouse effects.
Gases that satisfy the required refrigerant characteristics and that mainly contain hydrocarbons having a low boiling point have been searched for. However, since the choice of gases is restricted, it is difficult to find a single gas that satisfies these requirements. Consequently, two or more gases are mixed to adjust the characteristics of the refrigerants.
However, azeotropic refrigerant mixtures showing a constant boiling point are restricted in both combination and composition. Unlike single-component refrigerant gases that are widely used, many of the refrigerant mixtures composed of two or more components show a non-azeotropic characteristic.
These non-azeotropic refrigerant mixtures are different from refrigerants of single components or azeotropic refrigerant mixtures in that they can be given required intermediate characteristics deduced from properties of individual gases by selecting the ratio of these gases. On the other hand, since boiling points and dew points of these refrigerant mixtures are separated from each other, the composition in a gas phase and the composition in a liquefied or condensed phase are different from each other under conditions in which both a liquid phase and a gas phase are present. Therefore, during the condensing processes, these refrigerant mixtures cannot be condensed at constant temperatures and pressures, resulting in unstable operation of the refrigerating systems.
In order to resolve these problems, for example, Japanese Unexamined Patent Application Publication No. 51-83258 and Japanese Examined Patent Publication No. 5-45867 disclose refrigerating systems using non-azeotropic refrigerant mixtures. The temperature and pressure in each refrigerating system are controlled through an expansion valve depending on the evaporating pressure of the non-azeotropic refrigerant mixture and the corresponding saturation temperature, and alerting means is operated when the controlled conditions deviate from predetermined ranges. Specifically, the latter discloses heat exchange between a lower-temperature inlet refrigerant in a path from an evaporator to a compressor and a pressured refrigerant in a path from the compressor to the evaporator.
Each of these refrigerating systems uses a combination of refrigerant components, i.e. refrigerant R-22 having a low boiling point and refrigerant R-114 having a high boiling point. Since the respective normal boiling points are −40.8° C. and 3.85° C., the difference between the dew point and the boiling point specific to the non-azeotropic refrigerant mixture is large. Therefore, problems such as absorption of the liquefied refrigerant by the compressor occur. Such a problem is avoided by the control of the refrigerating system.
In Japanese Unexamined Patent Application Publication No. 8-166172, all of refrigerant components used in examples are fluorocarbons, i.e. R-32, R-125, and R-134a, and their normal boiling points are −51.7° C., −48.5° C., and −26.5° C., respectively. A non-azeotropic refrigerant mixture composed of these refrigerants is not liquefied until the mixture is cooled to a temperature significantly lower than room temperature. Consequently, a refrigerating system including a compressor, a condenser, a receiver, a decompressor, and an evaporator is further provided with a heat exchanger exchanging heat between the refrigerant flowing from the condenser to the receiver and the refrigerant flowing from the evaporator to the compressor.
The difference in boiling point between these refrigerants is small, namely, the above-mentioned problem is avoided by using a refrigerant mixture having a small difference between a dew point and a boiling point. In the condition in which both a gas phase and a liquid phase are present in the refrigerating system, only the liquefied non-azeotropic refrigerant mixture is separated by the receiver and is sent to the evaporator. Furthermore, gas-liquid separation is performed in suction piping in order to prevent hydraulic compression caused by mixing the liquefied refrigerant with a refrigerant gas that enters the compressor.
However, such a system construction is complicated. In addition, since the gaseous and liquid non-azeotropic refrigerant mixtures have different compositions when both a gas phase and a liquid phase are present, the system construction such that the gas-liquid separation is performed adversely affects a steady state control.